A reference voltage is an indispensable parameter in the design of an integrated circuit, and a bandgap reference voltage circuit is a solution for generating the reference voltage. The reference voltage (or current) generated by the bandgap reference voltage circuit should be independent of the process, voltage, and temperature (PVT) of the integrated circuit. For the limitation of the structure or size, the conventional bandgap reference voltage source circuit can only provide a reference voltage of about 1.25 V. When the voltage provided by a chip power supply is lower than 1.25 V, the operation of the conventional bandgap reference voltage source circuit becomes rather difficult.
A core part of a current-mode bandgap reference voltage source circuit provided in the prior art is as shown in FIG. 1. The principle that the current-mode bandgap reference voltage source circuit can provide a reference voltage is briefly analyzed in the following.
In the circuit shown in FIG. 1, three Positive Channel Metal Oxide Semiconductor (PMOS) transistors P1, P2 and P3 are of the same size, and resistances of a branch resistor R1 and a branch resistor R2 on two resistor branches are equal. A negative feedback network formed by an operational amplifier Amp enables a voltage Va at a node A to be equal to a voltage Vb at a node B (the nodes A and B are respectively connected to two input ends of the operational amplifier Amp, that is, Va and Vb are respectively equal to voltages of the input ends of the operational amplifier Amp). Because gates of P1, P2 and P3 are connected to an output end of the operational amplifier Amp, a relation of currents I1, I2, and I3 flowing towards drains of P1, P2 and P3 is I1=I2=I3. Furthermore, because Va=Vb, and R1=R2, I1b=I2b, and further, because I1=I2, I1a=I2a. 
It is known from the circuit structure shown in the figure and the circuit law that:
ΔVf=Vb−Vf2, Vb=Va, and Va=Vf1, so ΔVf=Vf1−Vf2=VT×lnN, where VT is a conduction voltage of each diode of N parallel-connected diodes in the figure;
I2a=ΔVf/R3;
I2b=Vf1/R2;
I2=I2a+I2b; 
I2=I3, so I3=I2a+I2b; 
the reference voltage output by the circuit is Vref=R4×I3=R4×(I2a+I2b)=R4×(ΔVf/R3+Vf1/R2).
Because Vf1 has a negative temperature coefficient, and ΔVf has a positive temperature coefficient, a reference voltage Vref independent of the PVT may be obtained by selecting appropriate R2 and R3, and reference voltages Vref having different values may be output by adjusting the resistance of the resistor R4.
A starting circuit is further required for normal operation of the current-mode bandgap reference voltage source circuit shown in FIG. 1. FIG. 2 shows a current-mode bandgap reference voltage source circuit having a starting circuit in the prior art. As shown in FIG. 2, before the circuit operates normally, a high level starting signal is first added on a gate of a Negative Channel Metal Oxide Semiconductor (NMOS) transistor shown by a dashed line block, the conduction of the NMOS transistor lowers a gate voltage V1 of the P3 transistor, and the circuit exits a zero current state, and enters an operation state.
Besides the zero current state and the operation state, the current-mode bandgap reference voltage source circuit shown in FIG. 1 may also have a middle state, that is, a weak current conduction state. The so-called weak current conduction state refers to that after the current-mode bandgap reference voltage source circuit exits the zero current state, if the voltage Va at the node A and the voltage Vb at the node B are too low to conduct the diode B1 and N parallel-connected diodes B connected in parallel with the resistor R1, the current only flows through the resistor branches, and no current flows through a branch where the diode B1 is located and a branch where the resistor R3 is located.
In the weak current conduction state, the operational amplifier Amp may still normally operate, and the voltage Va at the node A and the voltage Vb at the node B are still equal. However, because I2a is zero, according to the foregoing analysis of the current-mode bandgap reference voltage source circuit based on the circuit structure and the circuit law, in the weak current conduction state, the starting circuit provided by the prior art cannot enable the circuit to finally output a temperature independent voltage, so that the current-mode bandgap reference voltage source circuit may fail to start and cannot normally output a reference voltage.